This relates generally to semiconductor devices and processes, and more particularly to a structure and fabrication of hermetically sealed microelectromechanical system (MEMS) devices.
A wide variety of products collectively called microelectromechanical system (MEMS) devices are small, low weight devices on the micrometer to millimeter scale, which may have mechanically moving parts and often movable electrical power supplies and controls, or they may have parts sensitive to thermal, acoustic, or optical energy. MEMS have been developed to sense mechanical, thermal, chemical, radiant, magnetic, and biological quantities and inputs, and produce signals as outputs. Because of the moving and sensitive parts, MEMS have a need for physical and atmospheric protection. Consequently, MEMS are placed on or in a substrate and have to be surrounded by a housing or package, which has to shield the MEMS against ambient and electrical disturbances, and against stress.
A typical MEMS device integrates mechanical elements, sensors, actuators, and electronics on a common substrate. The manufacturing approach of a MEMS aims at using batch fabrication techniques similar to those used for microelectronics devices. MEMS can thus benefit from mass production and minimized material consumption to lower the manufacturing cost, while simultaneously realizing the benefits well-controlled integrated circuit processing technology.
Example MEMS devices include devices without moving parts and devices with moving parts. Examples of MEMS devices without moving parts are ink jet print heads mechanical sensors, strain gauges, pressure sensors with microphone membranes, and inertial sensors such as accelerometers coupled with the integrated electronic circuit of the chip. Among the MEMS devices with moving parts but without rubbing or impacting surfaces, are gyros, comb devices, resonators and filters. In other classes, the moving parts may impact surfaces, such as in digital mirror devices (DMDs), relays, valves, and pumps; or the moving parts may impact and rub surfaces, such as in optical switches, shutters, scanners, locks, discriminators, and variable electrostatic actuators (VEAs). In MEMS devices with moving parts, the mechanically moving parts are fabricated together with the sensors and actuators in the process flow of the electronic integrated circuit (IC) on a semiconductor chip. The mechanically moving parts may be produced by an undercutting etch or removal of a sacrificial layer at some step during the IC fabrication. Examples of specific bulk micromachining processes employed in MEMS sensor production to create the movable elements and the cavities for their movements are anisotropic wet etching and deep reactive ion etching.
While the fabrication of these MEMS devices can benefit from wafer-level processes, their packages do not have to be fully hermetic, i.e. impermeable to water molecules. Consequently, they may use sealants made of polymeric compounds typically used in adhesive bonding. In contrast, DMDs require substantially fully hermetic packages, because they may include torsion hinges, cantilever hinges, and flexure hinges. Each movable mirror element of all three types of hinge DMD includes a relatively thick metal reflector supported in a normal, undeflected position by an integral, relatively thin metal hinge. In the normal position, the reflector is spaced from a substrate-supported, underlying control electrode, which may have a voltage selectively impressed thereon by an addressing circuit. A suitable voltage applied to the electrode can electrostatically attract the reflector to move or deflect it from its normal position toward the control electrode and the substrate. Such movement or deflection of the reflector causes deformation of its supporting hinge which stores potential energy that mechanically biases the reflector for movement back to its normal position when the attracting voltage is removed. The deformation of a cantilever hinge comprises bending about an axis normal to a hinge axis. The deformation of a torsion hinge comprises deformation by twisting about an axis parallel to the hinge axis. The deformation of a flexure hinge, which is a relatively long cantilever hinge connected to the reflector by a relatively short torsion hinge, comprises both types of deformation, permitting the reflector to move in piston-like fashion.
An example DMD (digital mirror device) MEMS is a spatial light modulator such as a DLP™ DMD device available from Texas Instruments Incorporated. A typical DMD includes an array of individually addressable light modulating pixel element micromirrors, the reflectors of each of which are selectively positioned to reflect or not to reflect light to a desired site. To avoid an accidental engagement of a reflector and its control electrode, a landing electrode may be added for each reflector. A risk exists that a deflected reflector may stick to or adhere to its associated landing electrode. Such stiction effect (static friction to be overcome for enabling relative movement) may be caused by intermolecular attraction between the reflector and the landing electrode or by high surface energy substances adsorbed on the surface of the landing electrode and/or on the portion of the reflector which contacts the landing electrode. Substances that may impart such high surface energy to the reflector-landing electrode interface include water vapor or other ambient gases (e.g., carbon monoxide, carbon dioxide, oxygen, nitrogen) and gases and organic components resulting from or left behind following production of the DMD.
The problem of stiction has been addressed by applying selected numbers, durations, shapes and magnitudes of voltage pulses to the control electrode, or by passivating or lubricating the portion of the landing electrode engaged by the deformed reflector, and/or the portion of the deformed reflector which engages the landing electrode. Passivation is effected by lowering the surface energy of the landing electrode and/or the reflector through chemically vapor-depositing on the engageable surfaces a monolayer of a long-chain aliphatic halogenated polar compound, such as perfluoroalkyl acid. An effective method of passivation is to enclose a source of passivation, such as a predetermined quantity to time-released passivant material, in a closed cavity with the micromirrors at time of device manufacture.
Conventional hermetic packaging of MEMS devices usually involves a packaging process that departs from the processes normally used for non-MEMS device packaging. MEMS hermetic packaging is expensive because the package often includes a ceramic material, or a metallic or glass lid, and also because the package must be configured to avoid contact with moving and other sensitive parts of the MEMS device and to further allow a controlled or reduced atmosphere inside the package. However, the high package cost is in conflict with market requirements for many applications of MEMS devices, which put a premium at low device cost and, therefore, low package cost.
Further, the conventional fabrication of hermetic MEMS packages also encounters many technical challenges, such as those caused by potentially high temperatures in connection with welding of a hermetic lid to the package base. For example, a proposed package with a sealing process using a glass core involves temperatures considerably above 450° C., typically between 525 and 625° C. dependent on the sealing glass selected. These temperature ranges are a risk for the reliability of silicon integrated circuits and for proper functioning of many MEMS device components, and inhibit passivation and lubrication methods. Similar and sometimes even higher temperatures are involved, when packages use techniques such as anodic bonding and glass frit bonding.